One approach to achieving nanometer (nm)-scale feature sizes in semiconductor devices is to use shorter wavelengths of light when exposing photoresist layers. However, the difficulty in finding materials that are transparent below 193 nm exposure wavelength has led to the immersion lithography process to increase the numerical aperture of the lens by use of a liquid to focus more light into the film. Immersion lithography employs a relatively high refractive index fluid between the last surface of an imaging device (e.g., KrF or ArF light source) and the first surface on the substrate, for example, a semiconductor wafer.
In immersion lithography, direct contact between the immersion fluid and photoresist layer can result in leaching of components of the photoresist into the immersion fluid. This leaching can cause contamination of the optical lens and bring about a change in the effective refractive index and transmission properties of the immersion fluid. In an effort to ameliorate this problem, use of a topcoat layer over the photoresist layer as a barrier between the immersion fluid and underlying photoresist layer has been proposed. The use of topcoat layers in immersion lithography, however, presents various challenges. Topcoat layers can impact, for example, process window, critical dimension (CD) variation, resist profile and device failure due to coating defects such as dewetting defects.
To improve performance of topcoat materials, the use of self-segregating topcoat compositions to form a graded topcoat layer has been proposed, for example, in Self-segregating Materials for Immersion Lithography, Daniel P. Sanders et al., Advances in Resist Materials and Processing Technology XXV, Proceedings of the SPIE, Vol. 6923, pp. 692309-1-692309-12 (2008). A self-segregated topcoat would theoretically allow for a tailored material having desired properties at both the immersion fluid and photoresist interfaces, for example, an improved water receding contact angle at the immersion fluid interface and good developer solubility at the photoresist interface.
Topcoats exhibiting a low receding contact angle for a given scan speed can result in water mark defects. These defects are generated when water droplets are left behind as the exposure head moves across the wafer. As a result, resist sensitivity becomes altered due to leaching of resist components into the water droplets, and water can permeate into the underlying resist. Topcoats having high receding contact angles would therefore be desired to allow for operation of immersion scanners at greater scan speeds, thereby allowing for increased process throughput. U.S. Patent App. Pub. Nos. 2007/0212646A1 to Gallagher et al. and 2010/0183976A1 to Wang et al. describe immersion topcoat compositions that include a self-segregating surface active polymer which allow for improved water receding contact angles.
The inventors have found that focusing on increasing topcoat receding contact angle and scan speed alone can prove detrimental to the formed devices. While greater topcoat polymer hydrophobicity can result in a higher receding contact angle, it can also result in increased occurrence of coating defects, for example, dewets in the form of spike-shaped discontinuities in the topcoat layer and surface roughness of the layer. It therefore would be desired to have a topcoat composition that would provide satisfactory contact angle and developer dissolution rate properties to allow for high scan speeds while also minimizing coating defects.
There is a continuing need in the art for topcoat compositions that address one or more problems associated with the state of the art, and for pattern-forming methods making use of such materials.